torstai 16. syyskuuta 2010

I will be... learning Cisco CCNA

This web page exists for people to see how I learned Cisco's CCNA material. The CCNA is a certification of qualification in the networking technologies, manufactured by Cisco.

The page is thus about learning your first (or possibly some the very
first) certificates regarding computer networking. Networks arose in the 1960s-1970s, when there were first tests with cabled Local Area Networks, or LAN. In addition to a physical medium, such as a copper conductor (wire), the computers need to also use a common protocol for transferring data.

Data can consist of files, and this suffices: it explains all. In a simple scenario of data there's no need to categorize the types of data, since all data can be regarded as consisting of single bits (0 or 1).

IP protocol and the TCP protocol are open standards, defined in Request for Comments and they root to the very early days of Internet. But besides protocols, which are only text (rules), you need actually physical hardware that does to the electricity (signal) what the protocol says. These devices may be manufactured by a variety of vendors, and one of the most famous one is Cisco.

Cisco hardware is known to be robust, logical, and industrial-strength, so you can deploy a box in an office, a factory, or several other places, and it will very likely work after years of use. Using advanced methodologies one can also build redundant networks where the failure of any single component does not involve network down-time.

I will be introducing new terms as we go; so for example, in the very beginning you will notice that I refer to a networked PC as 'host', 'machine', or 'PC'. They all mean the same. Note that a host is by no means limited to PCs (Personal Computers).

A host can be a mobile phone, Macintosh, a Unix machine, or any other computer device that can communicate with another device. Not all machines are compatible by default; but here lies the beauty of a standard protocol: it enables the machines to talk a common language, even though they wouldn't otherwise do it.

For example in reality, mobile phones communicate via radio waves to a base station; from there on, the base station communicates usually via fiber or copper line to the rest of the network. Thus a mobile base station is a special host, which exposes the communication into the "air" - or ether, as communications people often like to say.

The simplest network

A network in computer terms means two devices, capable of following the rules set forth by a protocol, to communicate with each other. Simplest way to build a network would be just plug a cord (cable) between two PCs. This would be a symmetric network of two hosts. The term 'host' comes from the fact that a PC is 'hosting' the code which contains the processing rules for the machine; and thus the host essentially guides signals from a port to another port.

Note, that just plugging the cord would not necessarily automatically enable communication between the machines.

The computers have to understand each other. This is where a protocol
comes to picture. There were a lot of different protocols, competing for popularity, in the 1970s-1990s. From then onwards, TCP/IP was the winner.

LAN, WAN, or what?

It's important to distinguish between different scale of networks. A
small, local area network is called LAN. These typically employ between
2 to a maximum of few hundred hosts.

The six topologies tell the principles of
connecting the machines: the most expensive way is the mesh, where
there is a connection from every host to every other. This includes
a lot of redundant cabling. A practical topology is the star, which is often the solution when Cisco routers or switches are used in an office.

WAN is a larger network, spanning over a city. When you're going outside the office, you need to leverage the local ISP's (network provider) metropolitan or global networks. For a pay, they take your IP packets, and route them to their target. Essentially, you get a virtual network, which looks like you had your hosts in the very same physical network, even though in reality, there might be thousands of kilometers between the branch offices and hosts.